The field of prosthetic devices has advanced greatly through the years. Many current lower-limb prosthetics use sensors, microprocessor controllers, and valves or other actuators to assist the user's gait motion, frequently through the use of a hydraulic piston system which facilitates one or more degrees of freedom of motion within the limb, e.g. a knee joint or ankle joint. The ideal amount of damping in a joint is not a fixed value for all patients, or even for all activities for one patient. For an instance, a heavier or more active patient may want more damping in a joint to provide the resistance they need for walking or running, while that high damping value may provide too much motion resistance for a lighter patient. Similarly, a particular patient may want to feel a high damping or stiffness value while walking for comfort and stability, but may want a very low damping value during an activity like bicycling to allow free range of motion of their joints. On an even shorter time scale, as a patient walks over uneven ground or transitions from flat ground to steps, etc., it may be advantageous to vary the damping characteristic of the limb from one step to the next.
Currently many prosthetics use a manually-adjusted hydraulic valve to adjust the flow rate of hydraulic fluid in a piston at the joint. This allows the user or prosthetist the capability to adjust the damping value of the prosthetic, but it is a relatively time-consuming process and requires manually turning a valve with a tool. Thus it is practical for addressing the patient-to-patient variability, but not the activity-to-activity or step-to-step variation.
Some other prosthetic devices use microprocessors and actuators to adjust the resistance valves on a step-by-step basis. This is an obvious improvement over the manually-turned device because adjustments can be made more frequently and can be controlled by software on the limb rather than requiring input by the user or a prosthetist. However, these systems require battery energy to drive the microprocessor and actuators, and thus have a limited life before the user must recharge or change their limb's battery.
Damping is the process of removing mechanical energy from a system. In the above systems this is done by heating up the hydraulic fluid as it is forced through a small orifice valve, and this energy is lost as waste heat. By adding energy harvesting to the limb and converting the mechanical energy into electrical energy which can be stored back in the limb's battery, the same goal of removing mechanical energy is accomplished in a constructive manner instead of a wasteful manner.
What is currently lacking in the art is a method for causing damping by diverting energy into a useful storage reservoir, rather than rejecting that energy to the surrounding environment. By adding energy harvesting and an intelligent control circuit to the limb and converting the mechanical energy of motion into electrical energy which can be stored back in the limb's onboard battery, the same goal of removing mechanical energy is accomplished in a constructive manner instead of a wasteful manner. The addition of an energy harvester increases the functionality of the limb by allowing automatic on-the-fly adjustment of the damping characteristics of the limb. By adding this energy harvesting capability it also is possible to partially or fully recharge the existing battery during use. This can greatly extend the time interval between complete recharges reducing the user's need to replace the battery or to plug the device in to recharge the battery. Indeed, if enough energy is harvested, it would not be necessary to recharge at all.